This invention is directed at an improved extraction zone for liquid-liquid contacting. More specifically, this invention is directed at an improved extraction zone having particular applicability to the removal of soluble contaminants from a feedstream.
Solvent extraction is well-known and has been used for many years for product separation. In the petroleum industry solvent extraction has been widely used for the removal of impurities from process streams, such as the removal of aromatic compounds from lube oil feedstocks. In liquid-liquid extraction one or more components in the liquid mixture are removed by intimate contact with another liquid which is selectively miscible either with the impurities or with the desired product. Liquid-liquid extractions may be carried out in a number of different ways, such as by batch, co-current or countercurrent extraction. Countercurrent extraction frequently is a preferred method for effecting the extraction, since it is continuous and since fresh solvent typically contacts the product just before the product exits from the extraction zone. Usually the solvent utilized is selectively miscible with the impurity to be removed but not miscible, or only slightly miscible, with the product. Countercurrent solvent extraction techniques are widely used in the petroleum industry for effecting product purification. In the manufacture of lube oils, the lube oil feedstock frequently is passed through a countercurrent extraction zone to remove product impurities, such as undesired aromatic components. Solvents frequently employed for extracting the aromatic components from the lube oil feedstock include phenol and N-methyl pyrrolidone (NMP).
When it is necessary to increase the throughput of a lube extraction unit, for example because of increased product demand or because lower yields from a poorer crude require a higher feed rate, the solvent recovery sections of the plant often can be expanded by conventional means such as more heat exchange, additional flash drums, larger pumps, or larger capacity control valves. The ability of the internals in the solvent contacting tower to handle the increased load then may become the limiting portion of the plant. Therefore, it would be desirable to provide internals for the countercurrent contacting which have higher capacity per unit of tower cross-sectional area than internals currently in use while retaining hydraulic stability and effective contacting for mass transfer. The design of such trays is complicated by the fact that several feedstocks of different density, viscosity and yield are often processed in the same extraction zone at different feed rates, temperatures and solvent treats. Moreover, for a given feedstock the flow rates vary considerably within the extraction zone from tray to tray, thus requiring a great deal of hydraulic fexibility for the trays. Furthermore, these systems are usually characterized by very low interfacial tension, so that while mixing and mass transfer are easy, subsequent separation of the phases by settling and coalescence is difficult. Thus, a problem encountered in the design of the extraction zones is minimizing excess mixing to thereby avoid emulsion formation and excess recirculation and turbulence.
Previously, efforts have been made to improve the extractive process primarily by improving extraction tower internals. U.S. Pat. No. 3,899,299 discloses a countercurrent extraction zone in which the less dense feedstock enters at the bottom of the extraction zone, while the more dense solvent enters at the top. A series of horizontally disposed, vertically spaced-apart trays are located in the zone. The less dense feed rising through the column flows under the tray, over a dam-like device and passes into cascade weir means located at substantially the same elevation as the tray. Perforations in the perforate plate of the weir means cause the feed to be broken into small droplets, which pass upwardly to the area beneath the next higher tray, where the droplets coalesce. This process of droplet formation and coalescence is repeated at each tray in the extraction zone. Simultaneously, solvent passes downwardly flowing generally across the top of each tray removing impurities from the droplets of feed rising through the solvent. It has been found that this design was not completely satisfactory, at relatively high feed rates per unit of tower cross-sectional area because the buildup of oil under each tray, particularly the bottom tray, resulted in a loss of lube oil entrained in the bottom extract stream, which limited extraction zone capacity.
U.S. Pat. No. 2,759,872 is directed at a liquid-liquid extraction zone in which each tray includes a rectangular riser having a series of partitions disposed beneath downcomers. The laminar flow from the rectangular risers is dispersed into droplets in the downwardly flowing heavy phase. This design is not desirable because the parallel baffles, which form the riser and discharge channels, must be very close together to achieve low velocity laminar flow by frictional resistance. These small channels are susceptible to plugging with dirt, scale and corrosion by-products. In addition, the velocity through the discharge baffles, which is necessary to provide the desired frictional resistance, may induce entrainment of the heavy phase in the lighter phase. Moreover, since restrictive orifices are not used to reduce pressure drop, the riser height above the tray must be relatively great for an effective hydraulic seal without excessive recirculation of the heavy phase.
U.S. Pat. No. 2,791,537 discloses a liquid-liquid contacting device in which both oil and solvent pass co-currently through the V-notches of an underflow weir, thence up to a mixing zone provided with packing material. However, it has been found that the use of V-notches may result in unstable oil flow. Also, the passage of all the solvent and oil through packing material leads to gradual fouling of the packing with scale, corrosion byproducts, etc., leading to periodic shutdown of the equipment to clean the packing.
U.S. Pat. No. 2,861,027 discloses an extraction zone in which solvent and feed flow across the tower in a countercurrent extraction process. This patent is directed at reducing hydraulic instability, but does not disclose a cascade weir means as the device to accomplish reduced instability. Rather than reducing excess mixing energy in minimize emulsion formation, this design actually may encourage excessive mixing by providing two dispersion devices on each tray. This design is not well suited for low interfacial tension systems, such as lube oil and N-methyl pyrrolidone systems, where excess mixing hinders the separation.
U.S. Pat. No. 2,721,790 discloses a liquid-liquid contacting device utilizing perforate partitions and a coalescence means across the entire diameter of the extraction zone. This patent has the disadvantage inherent in a simple perforate plate of little flexibility in flow rate. Moreover this patent does not disclose a cascade weir type design for intimate contact of the solvent and feed. The use of a continuous coalescence means across the entire diameter of the vessel and at each stage of the extraction zone may be excessive and may lead to premature shut-down of the extraction zone due to fouling of the coalescense means which would hamper fluid flow through the vessel.
U.S. Pat. No. 2,520,391 discloses a liquid-liquid contacting device in which baffles are disposed perpendicular to the trays to prevent vigorous agitation of the liquid. This patent utilizes modified bubble caps for contacting. A gas is introduced into the system or produced in situ by boiling to facilitate intimate liquid-liquid contacting. This design is not advantageous because of the necessity of introducing or producing a gas in the extraction zone. Also the high agitation produced by the gas is not suitable for a low interfacial tension system.
U.S. Pat. No. 2,669,505 describes a tower comprising contacting plates for a liquid-liquid extraction tower in which both the solvent and feed pass through perforate plates countercurrently and then flow cocurrently to the next stage. This design is not preferred because there is no means to regulate the number of orifices in the perforate plate to the varying flow rates encountered in a lube oil extraction zone.
European Patent Publication No. 20,094 describes an extraction process in which coalescing surfaces are disposed in the flow path of the extract phase prior to its exit from the extraction zone. This patent publication does not disclose an arrangement of coalescing surfaces which would permit the extract phase to contact the coalescing surfaces when clean and to by-pass the coalescing surfaces when they become fouled.
Accordingly, it is desired to provide a countercurrent cascade weir type extraction zone which is flexible and operable without instability or flooding at higher through-put rates than the conventional cascade weir extraction zones noted above.
It also is desirable to provide an extraction zone design which is readily adaptable to existing facilities, which have low capacity internals.
It is also desirable to provide an improved countercurrent, extraction zone design which is not prone to plugging and does not require the addition of any extraneous fluids to promote mixing of the solvent and feed.
The present invention is directed at an improved countercurrent, cross-flow cascade weir type extraction zone particularly useful for liquid-liquid systems which form relatively stable emulsions. The present invention comprises a perforate cascade weir means elevated with respect to its associated tray and a controlled riser means for conveying the phase to be dispersed to the cascade weir means. The weir means typically may be elevated from about 5% to about 50% of the tray spacing in the extraction zone above its associated tray. Calming baffles, and/or coalescence means also may be added for improved performance.